In aircraft with multiple fuel tanks it is common practice to automatically transfer fuel between individual tanks during a flight as the fuel is used. Whilst the control of the fuel transfers will depend on the amount of fuel in the various tanks, other factors may also be taken into consideration. One such factor is the gross weight centre of gravity (GWCG) of the aircraft. As fuel is used the GWCG can move fore or aft along the central axis of the aircraft. If the GWCG is allowed to move beyond certain limits the ability of the pilot to control the aircraft can be impaired. The transfer of fuel between different tanks in the aircraft is used to control the movement of the GWCG and prevent the fore and aft limits being exceeded.
Some aircraft have one or more of the fuel tanks located towards the rear of the aircraft, for example within the horizontal stabilisers of the tail section or towards the rear of the cargo area. As this fuel tank is located at an extremity of the aircraft the weight of fuel held within it has a large impact on the GWCG of the aircraft as a whole.
Ordinarily the GWCG of the aircraft is calculated using the mass of fuel in each of the individual fuel tanks, the fuel mass being given by a fuel quantity indication (FQI) for each fuel tank. The transfer of fuel is controlled based on this calculated value, amongst other values, to maintain the GWCG within the fore and aft limits. However, in the event of a failure of one or more of the FQIs, and in particular the FQI for the trim tank, it is not possible to directly calculate the GWCG based on the reported mass of fuel and therefore not possible to directly control the transfer of fuel to control the GWCG on this basis.